The present invention relates to a method for correcting a controlled variable for the control of the air-fuel ratio or ignition timing of an internal combustion engine on the basis of the differential of the quantity of suction air of the engine measured by means of thermal type flowmeter.
In controlling the air-fuel ratio (hereinafter referred to as A/F ratio) of an internal combustion engine with high accuracy, it is necessary to obtain an accurate value of the suction air quantity of the engine. It is generally known that the suction air quantity is measured by using a thermal type flowmeter as is disclosed in U.S. Pat. No. 4,089,214.
As the thermal type flowmeter is a kind of mass flowmeter which can measure the mass of airflow, it can accurately measure the quantity of airflow without consideration of the change of temperature or atmospheric pressure. Having good responsiveness without including any mechanical moving parts, moreover, the thermal type flowmeter enjoys an advantage such that it is free from any output errors attributed to mechanical vibrations. Also for this reason, the thermal type flowmeter is suited for the measurement of the suction air quantity of an engine.
However, an output representing the suction air quantity actually measured by means of the thermal type flowmeter is subject to errors attributed to other causes than the aforesaid vibrations, such as pulsation of the suction air as the primary cause. The output errors are expressly noticeable when the engine is in a steady-state, high-load operation mode. Therefore the accurate control of the A/F ratio cannot be obtained, as shown in FIG. 1, because A/F ratio is controlled on the basis of the output of the thermal type flowmeter. In FIG. 1, the ordinate represents A/F ratio error (%) compared with the target value of the A/F ratio, while the abscissa represents the engine speed. As seen from FIG. 1, the A/F ratio greatly deflects from the target value to the rich side in a low engine speed range of about 2,000 rpm and less. In a medium or high engine speed range over 2,000 rpm, on the other hand, the A/F ratio deflects from the targer value to the lean side. This indicates that the thermal type flowmeter delivers an output value representing a suction air quantity greater than the true value in the low engine speed range, affected by the pulsation of the suction air, and that the flowmeter delivers an output value representing a suction air quantity smaller than the true value in the medium or high engine speed range. Accordingly, if the fuel injection quantity of the engine is controlled by the use of the thermal type flowmeter which is subject to such output errors, the drivability and emission performance of the engine will be lowered, causing the catalytic converter to overheat and at last, damage to the engine.
A thermal type flowmeter for a suction air measuring system in an internal combustion engine stated in Japanese Patent Disclosure No. 18721/81 is generally known as a measure to counter the influence of the pulsation of the suction air which causes the aforesaid output errors. In this conventional measuring system, the thermal type flowmeter is attached to an air by-pass for by-passing the suction pipe of the engine, thereby preventing the pulsation of the suction air produced in the suction pipe from affecting the output of the flowmeter. However, this system is subject to a drawback such that the engine is complicated in structure requiring the air by-pass attached to the suction pipe. Moreover, the thermal type flowmeter can measure only the quantity of suction air which flows through the air by-pass, and cannot directly measure the quantity of suction air which actually flows through the suction pipe. Thus, the thermal type flowmeter of this system cannot accurately measure the suction air quantity, exerting the aforementioned bad influence on the engine.